GH-82805: Fix handling of single-dot file extensions in pathlib (#118952)

pathlib now treats "`.`" as a valid file extension (suffix). This brings
it in line with `os.path.splitext()`.

In the (private) pathlib ABCs, we add a new `ParserBase.splitext()` method
that splits a path into a `(root, ext)` pair, like `os.path.splitext()`.
This method is called by `PurePathBase.stem`, `suffix`, etc. In a future
version of pathlib, we might make these base classes public, and so users
will be able to define their own `splitext()` method to control file
extension splitting.

In `pathlib.PurePath` we add optimised `stem`, `suffix` and `suffixes`
properties that don't use `splitext()`, which avoids computing the path
base name twice.